1. Field of the Invention
The present invention relates to a method of calibrating the forming rollers of an Assel-type rolling mill and, more particularly, to a method of calibrating an Assel mill for rolling thin-walled tubes of pre-pierced hollow bodies about a mandrel using at least three generally conical rollers which are circumferentially spaced about the mandrel.
2. Description of the Prior Art
The Assel rolling method, developed some 60 years ago by Walter Assel, is particularly well suited for producing or forming roller bearing tubes and thick-walled turned part tubes having a diameter to wall thickness ratio of approximately 16:1. The Assel rolling method has over time been further developed through permanent improvements into a powerful stretching method. The Assel method is used in the manufacture of tubes having medium and strong wall thickness and, in particular, tubes requiring flawless surfaces and close tolerances, as for example roller bearing steel tubes. An Assel mill operates according to the principle of piercing around mandrel bars, employing three generally conical rollers that are mounted circumferentially about a mandrel bar so as to be inclined relative to the rolling axis of the mandrel. The three rollers are also circumferentially staggered or spaced apart relative to one another by approximately 120.degree.. Furthermore, the rollers are vertically adjustable relative to their axis of rotation or roll so that a plurality of tube diameters can be produced on one Assel mill.
A forming roller of an Assel mill consists essentially of a conical entrance, a working shoulder, a smoothing part and a rounding part; the major forming work is carried out by the working shoulder of the roller. By using at least three generally conical forming rollers, the Assel method advantageously guides the rolling material and obviates the need for separate rolling material guide disks such, for example, as are required for the known Diescher process which uses two so-called arched rollers. The substantially smaller roll diameter of an Assel mill means that these mills can generally be notably smaller in size than corresponding Diescher rolling mills.
Like other known piercing methods, the Assel rolling method may permit the development of wall thickness irregularities that run in a helical line on the hollow ingot or tube and are known as "spirals." In a cross-section of the hollow body, these spirals act as an eccentricity, i.e. a deviation of the center points of the inner and outer circumferences relative to one another. In a longitudinal section of the hollow body, the spirals act as periodic and alternating thick and thin parts of the wall. Inadequate calibration of the forming rollers is the main cause of spiral formation on the hollow ingot or tube. For this reason, even though an Assel mill can maintain the strict wall thickness tolerances of .+-.4% to .+-.7% required for relatively thick wall tubes, tolerances in the case of thin wall tubes still leave much to be desired.
Another disadvantage of the Assel rolling method, as compared to other piercing processes, is the relatively low possible rolling speed, which restricts the capacity of the mill. The limits on rolling speed are formed by the maximum possible speed of the rolling material itself as well as the maximum possible transport angle. Too high a rolling material speed may lead to damage of the rolled tube, while too great a transport angle leads, in conventional roller calibration, to large spiral formation, i.e. poorer tube tolerances. Because it has not heretofore been and is not currently possible to significantly increase the speed of the rolling material and because the transport angle has been limited for acceptable resulting tolerance to approximately 7.degree., there has been no available known way to achieve further increases in rolling material speed. However, consideration was never given to the fact that, as discovered by applicant, the slope level of the bulge running in spiral fashion around the tube depends not only on the transport angle of the rollers but, also, on the tube diameter. The larger the tube diameter at the same transport angle, the larger the slope level of the spiral and the larger the difference between the thinnest and thickest portions of the walls. As a matter of principle, this also means that when tube diameters are small it is quite possible to roll with larger transport angles than previously possible if, for example, the slope level of the spiral is taken as a constant value. Furthermore, until now it has been possible to use the Assel method for only a limited purpose, namely, to achieve a maximum diameter/wall thickness ratio of approximately 12:1 to 16:1; in other words for thick-walled roller bearing tubes, turned part tubes, and the like. If larger diameter wall thickness ratios were selected, a triangulation effect occurred at the rear end of the tube which caused plugs as the tube left the forming rollers. This could only be prevented by timely ventilation of the forming rollers at the forming roller end.
If the Assel rolling method could also be successfully used to roll thin-walled tubes while maintaining acceptable surface and wall thickness tolerances, then the applications for this method could be broadened to include, for example, oil field tubes, boiler tubes and conduit tubes. The advantages of the Assel method compared, for example, to the Diescher rolling process could then be exploited. These advantages include good rolling material guidance by means of at least three rollers, good wall thickness tolerances in the tubes, low total investment costs and--because of the lower stress on the tubes in the rolling gap--better tube quality than in the Dieschef rolling process.